A drilling riser is made up of an assembly of tubular elements whose length generally ranges between 15 and 25 m, assembled by connectors. The weight of these risers borne by an offshore platform can be very great, which requires suspension means of very high capacity at the surface and suitable dimensions for the main tube and the connection fittings.
So far, the auxiliary lines: kill lines, choke lines, booster lines and hydraulic lines are arranged around the main tube and they comprise insertable fittings fastened to the riser element connectors in such a way that these high-pressure lines can allow a longitudinal relative displacement between two successive line elements, without any disconnection possibility however. Owing to these elements mounted sliding into one another, the lines intended to allow high-pressure circulation of an effluent coming from the well or from the surface cannot take part in the longitudinal mechanical strength of the structure consisting of the entire riser.
Now, in the perspective of drilling at water depths that can reach 3500 m or more, the dead weight of the auxiliary lines becomes very penalizing. This phenomenon is increased by the fact that, for the same maximum working pressure, the length of these lines requires a larger inside diameter considering the necessity to limit pressure drops.
Document FR-2,799,789 aims to involve the auxiliary lines, kill lines, choke lines, booster lines or hydraulic lines, in the longitudinal mechanical strength of the riser. According to this document, a riser pipe element (commonly referred to as joint or section) comprises a main tube, connection means at both ends, at least one auxiliary line length arranged substantially parallel to the main tube. The auxiliary line length is secured by both ends to the connection means of the main tube so that the longitudinal mechanical stresses undergone by the connection means are distributed within the tube and the line.
One difficulty in achieving the riser according to document FR-2,799,789 lies in the means for fastening the auxiliary line length to the main tube. The tensional stresses undergone by the auxiliary line length are transmitted by these fastening means. Assembly and building constraints require a minimum distance between the axis of the main tube and the axis of the auxiliary line. This distance acts as a lever arm for the tensional stresses transmitted to the auxiliary line. As a result of the tensional stresses associated with the lever arm, the fastening means are subject to flexural deformations that can hinder smooth operation of the riser or require penalizing mechanical designs.
The present invention provides a riser built according to an alternative principle to the one disclosed by document FR-2,799,789. According to the present invention, the auxiliary lines as a whole take part, jointly with the main tube, in the taking up of the longitudinal stresses applied to the riser.
Document WO-2007/039,688 describes linking means between the auxiliary lines according to the principle of connectors coaxial to said auxiliary lines, of bayonet locking system or screw nut type. However, these systems notably have the drawback of a relatively large overall diameter, which adds to the diameter of the connector of the main line, considering that the resistance elements have to be arranged concentrically to the fluid passage.